1. Field of the Invention
The invention relates to an electromagnetically driven valve control apparatus and, more particularly, to a control apparatus for electromagnetically driven valves that are used as intake valves and exhaust valves of an internal combustion engine.
2. Description of the Related Art
An electromagnetically driven valve is disclosed in, for example, Japanese Patent Application Laid-Open No. HEI 10-252426. The electromagnetically driven valve has a valve body that functions as an intake or exhaust valve of an internal combustion engine, and an armature fixed to the valve body. The valve body and the armature are displaceable in the directions of an axis of the valve body.
A first electromagnetic coil and an upper spring are disposed above the armature. A second electromagnetic coil and a lower spring are disposed below the armature. During a stop of the internal combustion engine, the armature is held at a neutral position, that is, an intermediate position between the first electromagnetic coil and the second electromagnetic coil, by the urging force from the upper and lower springs. The first electromagnetic coil and the second electromagnetic coil separately produce electromagnetic forces to attract the armature, upon receiving exciting current.
The armature and the valve body of the electromagnetically driven valve described above can be displaced in a valve opening direction by supplying exciting current to the first electromagnetic coil. The armature and the valve body can be displaced in the valve opening direction by supplying exciting current to the second electromagnetic coil. Therefore, according to the conventional electromagnetically driven valve, the exhaust or intake valve can be repeatedly opened and closed at arbitrary timing by supplying exciting current to the first electromagnetic coil and the second electromagnetic coil at suitable timing.
In an internal combustion engine equipped with electromagnetically driven valves as described above, the supply of exciting current to the first and second electromagnetic coils of each electromagnetically driven valve is stopped immediately upon turning off an ignition switch to stop the engine operation. As a result, an attraction force acting on the armature of each electromagnetically driven valve discontinues, so that each armature is held at a neutral position, that is, an intermediate position between the first electromagnetic coil and the second electromagnetic coil, due to urging force from the upper and lower springs. When the armatures of the electromagnetically driven valves are at the neutral position, the exhaust valves and the intake valves are held in a half-open state. Thus, according to the conventional electromagnetically driven valves, the exhaust valves and the intake valves are held in the half-open state after the ignition switch is turned off.
Normally, during an operation of an internal combustion engine and immediately after the engine operation stops, the pressure in the intake pipe becomes negative and lower than the pressure in the exhaust pipe. Therefore, with the above-described electromagnetically driven valves, since the exhaust valves and the intake valves are held at the half-open position upon the turning-off of the ignition switch, exhaust gas having a low oxygen concentration flows into the intake pipe via the exhaust and intake valves immediately after the internal combustion engine stops. If exhaust gas having a low oxygen concentration exists in the intake pipe, there is a possibility that when the internal combustion engine is to be started again, degraded engine starting characteristics may result from insufficient oxygen.